The relative importance of the sexual cycle in each pathosystem can vary depending on factors that include climate, host, cropping practices, patterns of pathogen migration, and the inherent biology of each species. This chapter describes the biology of sexual reproduction in phytopathogenic oomycetes and its role in disease. These infect both mono- and dicotyledonous crops, ornamentals, and native plants, causing foliar blights or root, crown, or fruit rots. Plant pathogens infect both mono- and dicotyledonous crops, ornamentals, and native plants, causing foliar blights or root, crown, or fruit rots. Various factors are reported to stimulate germination including plant extracts, light, carbon dioxide, and alternating temperature and wetness regimes, but the requirements do not seem to have universal effects on different oomycetes. Potatoes, as well as tomatoes and several other Solanum spp., are hosts of the heterothallic species P. infestans. Importantly, this inoculum would typically need to travel from their point of origin to distant potato fields before significant amounts of disease could occur, slowing epidemic progression. Nevertheless, there is evidence that sexual reproduction has occurred based on the appearance of recombinant genotypes. An interesting picture has emerged in which host, pathogen genotype, and environment interact to determine the importance of sexual reproduction. While sexual reproduction challenges efforts to control oomycete pathogens, better knowledge of the mechanisms of oospore formation and germination could lead to new management strategies. Soil populations of oospores might also be reduced by applying compounds stimulating oospore germination near the end of a growing season.

Light and scanning electron microscopy of P. infestans oospores. Note in the left image the thick inner oospore wall, which has retracted from the external layer of the oosphere, giving the appearance of an endospore. Shown are the antheridium (A), outer oospore wall (OOW), oogonial wall (OW), and ooplast (OP). Reprinted from Fungal Genetics and Biology (47) and Nature Reviews Microbiology (48) with permission of the publishers.

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10.1128/9781555815837/f0447-02.gif

Figure 27.2

Light and scanning electron microscopy of P. infestans oospores. Note in the left image the thick inner oospore wall, which has retracted from the external layer of the oosphere, giving the appearance of an endospore. Shown are the antheridium (A), outer oospore wall (OOW), oogonial wall (OW), and ooplast (OP). Reprinted from Fungal Genetics and Biology (47) and Nature Reviews Microbiology (48) with permission of the publishers.

Selfing and outcrossing by homothallic oomycetes. Oospores produced within a lesion caused by a single strain are necessarily selfs, but hybrids can form when plants are coinfected by two strains. The structures shown are representative of Pythium, where both monoclinous and diclinous antheridia can form. Note that the oospores illustrated are “paragynous,” in which the antheridium (A) contacts the side of the oogonium (O). This contrasts with the “amphigynous” oospores shown in Fig. 27.2 for P. infestans, in which the oogonium grows through the antheridium.

10.1128/9781555815837/f0448-01_thmb.gif

10.1128/9781555815837/f0448-01.gif

Figure 27.3

Selfing and outcrossing by homothallic oomycetes. Oospores produced within a lesion caused by a single strain are necessarily selfs, but hybrids can form when plants are coinfected by two strains. The structures shown are representative of Pythium, where both monoclinous and diclinous antheridia can form. Note that the oospores illustrated are “paragynous,” in which the antheridium (A) contacts the side of the oogonium (O). This contrasts with the “amphigynous” oospores shown in Fig. 27.2 for P. infestans, in which the oogonium grows through the antheridium.

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